This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-70567 filed on Mar. 14, 2002.
1. Field of the Invention
The present invention relates to a flow meter for measuring flow rate of air and in particular to a flow meter applied to an intake air flow meter for an internal combustion engine.
2. Description of Related Art
Conventionally, a flow meter having a membrane airflow sensor is used for measuring a flow rate of intake air of an internal combustion engine. The airflow sensor is disposed in a passage carrying the air to measure.
An air filter is disposed in an intake system of the engine in order to eliminate particles such as sand, which are included in the intake air and have relatively large diameters. However, the intake air also includes particles that have relatively small diameters (hundreds of micrometers, for instance) and are difficult to eliminate with the air filter. In a case where the particles having relatively small diameters enter the passage, a sensing part of the flow meter might be damaged if the particles collide with the sensing part, for instance, at a speed of tens of meters per second. Specifically, if the sensing part is a membrane airflow sensor, the membrane of the airflow sensor is very thin. For instance, the thickness of the membrane is approximately 1 micrometer. Therefore, the membrane will be easily damaged if the particles collide with it.
A flow meter disclosed in U.S. Pat. No. 5,404,753 (DE4219454A1) or U.S. Pat. No. 6,422,070 B2 (DE4407209A1) is formed with a contraction part in an air passage in which a flow meter is disposed. The contraction part narrows the cross-sectional area of the passage gradually in a direction of the flow. The contraction part straightens the flow of the air flowing near a sensing part of the flow meter and carries particles included in the air in parallel with the sensing part, in order to reduce the influence of the particles to the sensing part.
In the above flow meter, the contraction part prevents the collision of the particles against the sensing part to some extent. However, the particles have some weight and inertia. Therefore, there is a possibility that the flow of the particles might not be straightened sufficiently compared with the airflow, depending on the shape of the contraction part. Accordingly, the particles whose flow is not straightened sufficiently might collide with the sensing part and might damage the sensing part.
A flow meter disclosed in U.S. Pat. No. 6,336,360 B1 (JP-A-11-248505) has a sensing part disposed in an air passage formed generally in a sigmoid shape. The air passage is formed with folded portions upstream and downstream of the sensing part in order to deflect the airflow and to inhibit the collision of the particles against the sensing part.
In the above flow meter, the particles bypass the sensing part to some extent, since the air passage has the folded portions. However, the effect of the folded parts is not enough to fully prevent the particles from colliding with the sensing part. As a result, the sensing part might be damaged by the particles, or the detection accuracy might be degraded because of adhesion of contaminants to the sensing part.
It is therefore an object of the present invention to provide a flow meter that prevents particles from colliding with a sensing part and prevents contaminants from adhering to the sensing part, whereby measuring air flow rate stably and accurately.
According to an aspect of the present invention, a flow meter is formed with a passage for introducing and carrying air to measure, and has a sensing part disposed in the passage. The flow meter is formed with a contraction part for narrowing the flow of the air flowing near the sensing part. The contraction part is formed on the inner surface of the passage so that the contraction part faces the sensing part. The contraction part is formed with a flat surface in parallel with the sensing part and at least upstream of the sensing part.
Since the flat surface of the contraction part is formed at least upstream of the sensing part, the air to measure flows along the flat surface of the contraction part and in parallel with the surface of the sensing part when the airflow enters the passage and flows near the sensing part. Meanwhile, the airflow is favorably straightened. Accordingly, the collision of the particles in acute angles against the sensing part is inhibited. Therefore, impulsive force generated when the particles collide with the sensing part is reduced, and the damage in the sensing part or adhesion of contaminants to the sensing part is inhibited. In addition, the airflow is favorably narrowed near the contraction part. Accordingly, the speed of the airflow is increased and the airflow is stabilized. As a result, the flow rate of the air is measured accurately.